Optical system for nonintermittent film projectors



' .LM UJJ Jan. 19, 1954 OPTICAL. SYSTEM Original Filed June 13, 1950 R.E. GRAHAM ET AL FOR NONINTERMITTENT FILM PROJECTORS 4 Sheets-Sheet 1PHOTO MUL T/PL/E CELL a7 TO MOVING c0/L- Mo ro ggs o u PHOTO DIFFERENCE60 R MUL 77PL/ER AMPLIFIER M RR RS CELL 2,

Y R. E. GRAHAM WVENTORS an MA 77'KE A TTORNE V Jan. 19, 1954 OPTICALSYSTEM Original Filed June 15, 1950 FIG. 2

E. GRAHAM ET AL FOR NONINTERMITTENT FILM PROJECTORS FIG. 4

C By 4 Sheets-Sheet '2 UPWARD DISPLACEMENT Jam/=- SL/T L WIDTH R. E.GRAHAM WVEVTORS F. MA TT/(E.

AT ORNEY 19, 1954 R. E. GRAHAM ETAL 2,666,357

OPTICAL SYSTEM FOR NONINTERMITTENT FILM PROJECTORS Original Filed June13, 1950 4 Sheets-Sheet 3 GUIDED EDGE OF FILM FIG. .54

RE. GRAHAM INVENTORS a E MATT/(E A T Tom/Er Jan. 19, 1954 GRAHAM ET AL2,666,357

OPTICAL SYSTEM FOR NONINTERMITTENT FILM PROJECTORS Original Filed June13, 1950 4 SheetS -Sheet 4 'R. E. GRAHAM c. F. MA TT/(E Al7 J MW ATTORNEV Patented Jan. 19, 1954 2,666,357 oPTicAL SYSTEM FOR NONI'NTERMITTENTFILM PROJECTORS Robert E. Graham,- Mattke, Fanwood,

Morristown, and Charles 1 N. L, assignors to Bell TelephoneLaboratories, Incorporated, New York,

N. Y., a corporation of N cw York Original application June Divided andthis 13, 1950, Serial application December 27, 1950, Serial No. 202,884

7 Claims.

This invention relates to optical systems and more specifically, by wayof example, to systems of this type suitable for use in a continuousprejec'tor' for motion pictures. This application is a division of anapplication of the same inventors, Serial No. 167,872, filed June 13,1950.

An object of the invention is to provide an improved optical system foran electro-optic'al arrangement whereby stationary and continuouslymoving optical patterns may be produced one from the other. v p

In the pending application of Charles F. Mattke, Serial No. 124,403,filed October 29, 1949, there is disclosed a special film projector bymeans of which the frames of a continuously moving motion picture filmare projected as sta-' tionary images on a viewing screen, and theillusion of scene or subject motion is produced as images of successiveframes are merged on the viewing screen. This projector compensates forthe motion of the film by employing a rotating frames or 60 fields persecond. The special projector disclosed in the Mattke ap-' plication,however, by virtue of its non-intermittent operation, avoids theconsequences of this dissimilarity. In one illustrative embodiment ofthis example of practice; the viewing screen is replaced by theluminescent screen of a cathode-ray tube, the cathode beam of which is 2the film from its proper instantaneous position. It is, therefore, anobject of the present in lower edge) of the sprocket holes. Thepresentinvention relates to the optical system used the arrangement justdescribed.

Fig. 1 shows in schematic form the general arrangement of thenon-intermittent projector;-

Fig. 2 shows the details of an illustrative em bodiment of the opticalsystem of this inventiontogether with a correcting servo;

Fig. 3 illustrates an arrangement of error detecting circuit used withthe optical system in accordance with the invention;

Fig. 4 is a plot of error amplifier output versus shown in Fig. 5;

Fig. 6' illustrates another embodiment of the invention in which bothhorizontal and vertical correction is efiec'ted; and

It will be instructive at this point to identify'the several basiccomponents of an illustrative em bodiment of the continuous filmprojector, as shown in Fig. 1, and the manner in which they cooperatewith each other to accomplish the desired result. The principalcomponents are a curved film gate l l with a radius of curvature R; arotating mirror drum I? which contains a crown of mirrors l3, whose axesare mounted parallel with the drum axle l4; and an errorcorrectingmirror Ill. The film l! is moved down at a constant rate through thegate H, where, in one exemplary arrangement of the projector,approximately two and one-half frames are illuminated by the lightsource It. To facilitate explanation of the operation of the device, letus assume temporarily that crown mirror l3 and correcting mirror Itremain stationary. Then the screen picture 19 will move upward at theframe rate of the film. This picture motion is canceled when therotation of the mirror drum l2 imparts the proper angular rotation tomirror It, and to each successive crown mirror, frame by frame. Thisdesired angular rotation is obtained by means of a gear train betweenthe mirror drum and film sprocket together with a cam and followeractuation of the individual crown mirrors about their respective axes.For a complete description of this arrangement, reference can be made tothe pending application of C. F. Mattke, which has already beenmentioned.

Because of perturbations of the film motion at the gate and mechanicalinaccuracies of the crown mirror drive, it is impossible to obtainprecisely the desired angular rotation, and it is thus the purpose ofthe servo-correcting mirror it to suppress the residual errors inpicture position due to these causes.

The technique by which this correction is accomplished is shown in Fig.2. Film sprocket holes 2i, which are used to locate the film during thephotographic processes of exposure and printing, can be made to serve asan accurate index of film position in deriving an error signal toactuate the correcting mirror. Images of the sprocket holes can beproduced for this purpose either by transmitting light through thesprocket hole and surrounding region of the film or by reflecting lightfrom this region. The transmission method is sensitive to variations infilm density around the border of the sprocket hole due to exposuredifferences or scratches. On the other hand, specular reflection fromthe hole border is relatively insensitive to these variations, and, inthe embodiment of the invention shown in Fig. 2, it is this techniquewhich is employed, although it is to be understood that the transmissionmethod is equally within the purview of the invention.

In accordance with the exemplary embodiment of the invention shown inFig. 2, the reflected image 22 is obtained by the use of an auxiliarylight source 23 on the crown side of the gate H. The light is directedto the sprocket hole areaof the film through a prism 26 and a lens 24.The smooth surface of the film forms a cylindrical mirror which reflectsthe light, except Where it impinges on the holes themselves, back to thecrown mirror l3, where it is deflected up through the lens 21. It isthen deflected by the correcting mirror it to a second prism 28 and lens28 at one side of the field, where the image 22 is directed upon thephotocell pick-up 3 I. This pick-' up consists of two photocells and isso located and masked (having apertures 32 and 33) that the reflectedlight which outlines the sprocket hole image 22 casts equivalent amountsof light upon each cell when the picture is in its proper posi tion, andin that case the two photocell signals 34 and 3B cancel one another atthe servo-ampliher 31.

Any vertical movement of the sprocket hole image will, however, castnon-equivalent amounts of light upon the two cells, and the output ofthe two cells will not then cancel each other. This unbalance in theoutputs of the two cells forms an error signal 38 which is amplified andsent to the two individual moving-coil driver units 39 and M which drivethe correcting mirror it, and thus the mirror is driven to return theimage to its proper position.

The error-detecting means may be located at the screen, rather than inthe projector itself. The error pick-ups may be adjusted not so thatthey necessarily pick up equal amounts of light when the picture is inits proper position, but so that their outputs are equal. That is, thebalance position can be so chosen that one photocell pickup receivesconsiderably more light than the other, but the gains of the photocellsare so adjusted as to provide identical outputs.

In Fig. 3, there is shown one such illustrative arrangement of thephotocell pick-ups. In this arrangement, the position of just the topedge of the sprocket hole image is monitored, instead of both the topand the bottom edges, as in the embodiment discussed above. In Fig. 3,as drawn, the slits Bi and 62 and the sprocket hole image 63 are shownin their relative positions for the correct picture position. Under thiscondition, slit 62 is bisected by the top edge of the sprocket holeimage 63, and slit 61 is entirely in the uniformly illuminated region.Obviously, since light reflected from the sprocket hole region is beingused, the image of the hole is dark, whereas the surrounding regions arebright. Thus, slit 62 is receiving half the total light received by slit6|. The gains of the two photomultipliers 64 and 66 are nevertheless soadjusted that equal respouses are obtained from the two cells under thiscondition. The differential amplifier ti subtracts the two equal inputsignals and thus produces zero output to the motors which drive thecorrector mirror. But when the sprocket hole image is displaceddownward, slit 62 receives more light than in the balanced condition,while the amount of light on slit BI is unchanged. This causes theproper polarity of output from the differential amplifier B1 to correctfor the image displacement. When the sprocket hole image is displacedupward, slit 62 receives less light than in the balanced condition, andif the displacement continues far enough, it will be completely in thedark. Since the light on slit Bl remains unchanged (at least until thetop edge of the sprocket hole image 63 reaches it) the output of thedifferential amplifier has the opposite polarity, and the imagedisplacement is similarly corrected.

With an arrangement of this sort, the output of difference amplifier 61varies with the upward displacement of the sprocket hole image from itsreference position approximately as shown in Fig. 4. Displacements inthat figure are plotted in terms of slit widths, and for purposes ofillustration, it is of interest to note that the widths of the slitswhich are at present extensively used are approximately 0.006 inch, theslits being separated by about 0.005 inch, while the sprocket hole imageis about 0.15 inch high.

Slight instantaneous brightness gradients occur in the image at certainrelative positions of the crown mirrors and the film, and if twomonitoring slits, at the top and the bottom of the sprocket hole image,respectively, are employed, this gradient creates a false unbalancebetween them. Preferably, however, the two slits 6! and 62 to becompared are very close together, so that mild brightness gradients donot produce appreciable unbalance.

For purposes of simplicity of exposition, the embodiments of theinvention which have thus for both vertical and horizontal errors ofposition, and Figs. 5 and 6 illustrate two exemplary arrangements foraccomplishing this.

The embodiment drawn in Fig. 5 is substantially the same as thatshown inFig. 2, with the double mirror system, one mirror 1| served. toeliminate vertical jitter while the other mirror 12 compensates forhorizontal errors. In the figure, the respective mirror axes are shownas- VV for mirror H and HH for mirror 12. Mirror H is driven by movingcoil driver units 9i and 92, and mirror 72 is driven by moving coilunits 93 and 94. The mirror assembly used for each component can, inaccordance: with the invention, be similar to that shown in Fig. 7 anddescribed below in connection with that figure. The error detecting unitconsists of two double slit assemblies 13 and 74-, with the necessaryphotocell hole, as illustrated in Fig. 5A. In. Fig. 6 there is shown anexemplary embodiment of the invention in which both horizontal andvertical correctionis effected by a single mirror 8! pivoted on a singlepoint and driven by two sets of driver units, one set for verticalcorrection about axes VV and the other set for horizontal correctionabout axes I-IH.

In order to illustrate the operation of the system, it will be helpfulto describe the mechanical design of the compensating mirror assembly ofone particular version which has actually been constructed and whichperforms extremely well, although it is obvious that many otherparticular designs, all within the scope of the invention, arethoroughly feasible and perhaps, in some situations, considerably to bepreferred.

In designing the specific model now under discussion, it was consideredimportant that, the

37, while high stiffness prevents the mirror from breaking up intohigher modes of vibration. In Fig. 7 there is shown one highly suitabletype of mirror which can be used in the specific embodiment of theinvention of the present discussion.

This mirror 5i can be formed by cutting an ellipse from a planoconvexlens blank and polishing" the fiat surface. This shape is well suitedfor handling the bundle of the light which strikes the mirror and itplaces most of the mirror mass near the axis of rotation, which is theminor axis of the ellipse. To illustrate the order of magnifactoryversion of the mirror which can be used in the representative embodimentof the invention under discussion has dimensions of three by four andone-half inches, with a maximum thickness of three-eighths inch at thecenter. In

that particular embodiment, a cast aluminum cradle 52 is firmly cementedto the back of the mirror for mounting purposes.

This cradle contains bosses upon which the moving coils of the driverunits are cemented. The cradled mirror is spring mounted on a stationaryframe 53, which supports the magnets 54, so that the voice coils areconcentric with the magnet plugs. The mirror, asstated above, is drivenby twoequal and opposite forces supplied by the two moving coil driverunits, which, in the particular embodiment being discussed, may comprisetwo General Electric S'545-D moving coil driver units or theirequivalents. The cradled mirror is supported on the stationary frame 53by means of two fiat springs, one on each side of the assembly. One endof each spring is clamped to the mirror cradle 52 and the other end tothe stationary frame, so that the support effectively consists of twospring steel cantilever beams. This type of suspension is by no meansthe only type which is feasible in the practice of the invention, but ithas been described as a convenient example becauseit is stable andfairly easy to construct. It is desirable in this sort of arrangementfor the mount ing constraint to have low stiffness about the axis ofrotation and a high stiffness to any other motion, and this can beaccomplished by using thin, narrow springs having a very short beamlength. The springs, in this particular embodiment, are installed in avertical position, so that the weight of the mirror is supported intension alone, and the movement of the mirror is restricted by stop pins55 which prevent the springs from deflecting beyond a safe range.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. In a film device, a light source, a screen, a film gate through whichfilm is continuously fed, a rotating mirror drum containing a crown ofmirrors, the axis of which is perpendicular to the direction oftransmission of the film image so that the film image is reflected bysaid crown variable position, and means for continuously varying theposition of said further mirror.

2. Ina film device, a light source, a screen, a film gate through whichfilm is continuously fed, a rotating mirror drum containing a crown ofmirrors, the axis of which is perpendicular to the direction oftransmission of the film image so that the film image is reflected bysaid crown of mirrors, said crown of mirrors comprising a plurality ofmirrors mounted on said drum for oscillation about individual axes lyingin the reflecting plane of each of said mirrors parallel to the axis ofsaid drum, a further mirror by which said reflected film image isfurther reflected to the screen, said further mirror having a variableposition, and means for continuously varying the position of suchfurther mirror about a stationary axis.

3. In a film device, a light source, a screen, a film gate through whichfilm is continuously fed, a rotating mirror drum containing a crown ofmirrors, the axis of which is perpendicular to the direction oftransmission of the film image so that the film image is reflected bysaid crown of mirrors, said crown of mirrors comprising a plurality ofmirrors mounted on said drum for oscillation about individual axes lyingin the refleeting plane of each of said mirrors parallel to the axis ofsaid drum, a pair of additional mirrors by which said reflected filmimage is further reflected to the screen, and means for continuouslyvarying the position of the two additional mirrors about axesrespectively at right angles to one another.

4. An optical system comprising a screen, a film gate through which filmis continuously fed, a rotating mirror drum containing a crown ofmirrors, the axis of which is perpendicular to the direction oftransmission of the film, said crown of mirrors comprising a pluralityof mirrors mounted on said drum for oscillation about individual axeslying in the reflecting plane of each of said mirrors parallel to theaxis of said drum, a further mirror in the light path between said crownof mirrors and said screen for reflecting an image on one of them to theother, said further mirror having a variable position, and means forcontinuously varying the position of said further mirror.

5. An optical system comprising a screen of a cathode-ray tube, a filmgate through which film is continuously fed, a rotating mirror drumcontaining a crown of mirrors, the axis of which is perpendicular to thedirection of transmission of the film, said crown of mirrors comprisinga plurality of mirrors mounted on said drum for oscillation aboutindividual axes lying in the refleeting plane of each of said mirrorsparallel to the axis of said drum, a further mirror in the light pathbetween said crown of mirrors and said screen for reflecting an image onone of them to 8 the other, said further mirror having a variableposition, and means for continuously varying the position of saidfurther mirror.

6. In a film device, a screen of a cathode-ray tube, a film gate throughwhich film is continuously fed, a rotating mirror drum containing acrown of mirrors, the axis of which is perpendicular to the direction ofmovement of the film so that the image on the screen is reflected bysaid crown of mirrors upon said film, said crown of mirrors comprising aplurality of mirrors mounted on said drum for oscillation aboutindividual axes lying in the reflecting plane of each of said mirrorsparallel to the axis of said drum, additional mirrors between the screenand the crown of mirrors by which the screen image is further reflectedto the film, said additional mirrors having a variable position, andmeans for continuously varying the position of said additional mirrors.

7. In a film device, a screen of a cathode-ray tube, a film gate throughwhich film is continuously fed, a rotating mirror drum containing acrown of mirrors, the axis of which is perpendicular to the direction ofthe film so that the image on the screen is reflected by said crown ofmirrors upon said film, said crown of mirrors comprising a plurality ofmirrors mounted on said drum for oscillation about individual axes lyingin the reflecting plane of each of said mirrors parallel to the axes ofsaid drum, additional mirrors between the screen and the crown ofmirrors by which the screen image is further reflected to the film, saidadditional mirrors having a variable position, means for continuouslyvarying the position of said additional mirrors, and photocell pick-upmeans positioned in the path of light from said mirror drum passingthrough said film.

ROBERT E. GRAHAM. CHARLES F. MA'ITKE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,541,218 Higginson June 9, 1925 2,155,013 Horton Apr. 18,1939 2,163,543 Clothier et al. June 20, 1939 2,278,781 Harrison Apr. '1,1942 2,563,892 Waller et al. Aug. 14, 1951 2,590,281 Sziklai et al. Mar.25, 1952 FOREIGN PATENTS Number Country Date 309,655 Great Britain Apr.18, 1929

